This invention relates to a system including a reference light source for calibration of an optical instrument on board a satellite in orbit.
In space it is almost inevitable that an onboard optical instrument is contaminated from various sources such as cosmic dust, radiation, gases jetted from thrusters for attitude control and gases evolved from adhesives used in the satellite, and it is impossible to accurately predict the degree of degradation of the optical instrument due to contamination. Therefore, it is useful to carry out periodic calibration of an onboard optical instrument with reference light radiated from an onboard light source. A very stable light source such as a halogen lamp is employed as the reference light source, but still there is a possibility that the quantity of light radiated from the light source slightly changes with the lapse of time. Therefore, a photodetector is provided in the vicinity of the reference light source in order to monitor the quantity of radiated light at any time when the calibration is performed.
The calibration unit includes optics to guide the reference light to the optics of the optical instrument to be calibrated. The optics of the calibration unit is also subject to contamination. Therefore, when the result of calibration seems to indicate degradation of the optical instrument, there is a possibility that the actual event is the contamination of the optics of the calibration unit. That is, it is very difficult to accurately judge whether the optics of the optical instrument to be calibrated is contaminated or the optics of the calibration unit.